feat(polka-storage-provider): submit PoSt on the pipeline

[th7nder]

Description

Write an explicit description about what the PR changes, and how you implemented
the change.

The quality of the description directly impacts the quality of the review
process for everyone. Adding relevant context (i.e. describing the reasons for
the change, adding links to relevant documents, etc) to your description ensures
that everyone is able to easily gather the context necessary to review your
change.

Closes #596.

Important points for reviewers

Add some additional resources that could help the reviewer with the
review. Like link to specs if implementing a protocol.

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
• Make sure that you described what this change does.
• If there are follow-ups, have you created issues for them?
  • If yes, which ones? / List them here
• Have you tested this solution?
• Were there any alternative implementations considered?
• Did you document new (or modified) APIs?